1. Field of the Invention
The present invention pertains to a one-step process for the formation of vinyl chloride polymers using a water-soluble initiator and a prehomogenized aqueous mixed emulsifier system.
2. Description of the Prior Art
The use of a straight chain surfactant, e.g., sodium lauryl sulfate, in the emulsion polymerization of vinyl chloride will generally result in the formation of vary small polymer particles unless the quantity of surfactant is carefully controlled by an incremental feed technique. One common way in which larger particles of polyvinyl chloride can be obtained is by first forming a "seed" particle of the polymer in an initial polymerization procedure, followed by a second polymerization or "seed growth" step. Some recently issued U.S. patents which relate to this seed growth technique are U.S. Pat. No. 3,383,346 to E. S. Smith, U.S. Pat. No. 3,642,740 to J. K. Pierce, Jr. and U.S. Pat. No. 3,755,225 to J. K. Pierce, Jr. et al. The effect of a mixed emulsifier system during only the second step of such a process has been studied in the scientific literature. J. Ugelstad et al. J. Polymer Sci., Symposium No. 42, 473-485 (1973).
In addition to the foregoing prior art, the presence of various alcohols, such as stearyl alcohol, as a component in a polymerization reaction medium has been studied by other investigators. For example, U.S. Pat. No. 3,324,097 to G. E. A. Pears and U.S. Pat. No. 3,654,248 to E. Iida et al., relate to a polymerization system wherein a homogenized vinyl chloride monomer is polymerized in the presence of an oil-soluble catalyst in a suspension polymerization procedure. When such a system is used a mixture of emulsion and suspension polymerized polyvinyl chloride particles are formed. The effect of a mixed emulsifier of surfactant and alcohol in a one-shot polymerization of styrene has also been studied: J. Ugelstad et al., Die Makromolekulare Chemie, Vol. 175, pp. 507-521 (1974); J. Ugelstad, J. Polymer Science, Polymer Letters, Vol. 11, pp. 503-513 (1973); and A. R. M. Azad et al., ACS Polymer Reprints, Vol. 16, No. 1, pp. 131-142 (April 1975).
Finally, a one-step emulsion polymerization process for forming vinyl chloride polymers is described in copending U.S. Ser. No. 740,990, entitled "Emulsion Polymerization of Vinyl Chloride Using Mixed Emulsifier System", filed on even date herewith in the name of Paul Kraft, wherein a mixed emulsifier system of a C.sub.12 -C.sub.18 straight chain alkyl or alkenyl surfactant, e.g., sodium lauryl sulfate, and a C.sub.16 -C.sub.20 straight chain alkyl or alkenyl alcohol and/or a saturated hydrocarbon having a chain length of greater than 18 carbon atoms e.g., cetyl alcohol is used. Although such a system gives a binodal distribution of polyvinyl chloride resins it is difficult to perform in a reproducible manner and can yield in successive runs products having vastly different particle size distributions.
It has not, however, been appreciated hitherto that a one-step polymerization procedure for vinyl chloride monomer can be achieved using a water-soluble initiator and a prehomogenized aqueous mixed emulsifier system in order to overcome the deficiencies associated with the prior technique of using a mixed emulsifier system. With use of the present invention a polydisperse distribution of polyvinyl chloride in the particle size range of from about 0.04 to about 1.1 microns is obtained with the bulk being in the range of from about 0.6 to about 0.7 microns. It has a generally larger amount of larger particles as compared to the prior technique and the resin blend has generally superior mechanical stability and thermal stability thereby making it more commericially desirable as a plastisol or organosol resin. The process is much more capable of yielding reproducible particle size results in successive runs than is the prior art process.